Flow-through electrochemical cells are used as detectors for a variety of separation systems including chromatographic and ion chromatographic systems. Dionex Corporation sells such electrochemical cells under the trademarks ED40 and ED50 cells. Such cells include an amperometric working electrode in the form of a cylindrical wire embedded into a plastic block with the tip of the wire exposed to a sample flow-through channel, typically enclosed by a plastic gasket held in place under compression. These working electrodes are somewhat complicated and expensive to manufacture. After a period of use, the electrode must be replaced or reconditioned by laborious polishing or other methods which can lead to a lack of reproducibility of the detector output.
Thin film disposable electrodes have been used as in vitro test electrodes and as in vivo implantable monitoring electrodes in a variety of applications. See, for example, Michel, et al. U.S. Pat. No. 5,694,932; Dahl, et al. U.S. Pat. No. 5,554,178; Saban, et al. U.S. Pat. No. 6,110,354; Krause, et al. U.S. Pat. No. 4,710,403; Grill, Jr., et al. U.S. Pat. No. 5,324,322; Kurnik, et al. U.S. Pat. No. 5,989,409; Diebold, et al. U.S. Pat. No. 5,437,999; Kuennecke, et al. WO 99/36786; Bozon, et al., Electroanalysis 13:911-916 (2001); Soper, et al., Analytical Chemistry 72:642A-651A (2000); Lindner, et al., Analytical Chemistry 72:336A-345A (2000); Bagel, et al., Analytical Chemistry 69:4688-4694 (1997); Madaras, et al., Analytical Chemistry 68:3832-3839 (1996); and Marsouk, et al., Analytical Chemistry 69:2646-2652 (1997). However, none of the disposable electrodes described in these references are suggested for use in a flow-through electrochemical cell. Such cells have unique requirements such as the requirement of minimal contribution to peak broadening and reference potential being independent of sample composition.
The minimal contribution to peak broadening is predominantly determined by a low value of xe2x80x9cchromatographic dead volume.xe2x80x9d
The independence of reference potential from solution composition is realized only in xe2x80x9ctruexe2x80x9d reference electrodes e.g. calomel or Ag/AgCl equipped by a special type of electrolytic connection known as xe2x80x9csalt bridge.xe2x80x9d A typical salt bridge is a cylindrical container filled with a 3 M KCl solution. The conductive connection to the reference half cell on one side and to the sample on the other side is realized using ion permeable diaphragms.
All existing microfabricated cells employ either xe2x80x9cpseudoxe2x80x9d reference electrodes (e.g. palladium) or reference half cells without salt bridges. The latter types of reference electrodes rely on a constant concentration of chloride ions in a measured sample. Achieving such constant concentration of chloride ions is not practical under chromatographic conditions.
There is a need to provide a disposable and readily removable amperometric working electrode for a flow-through electrochemical cell which is less expensive to construct and is replaceable, thus avoiding the potential lack of reproducibility incurred in reconditioning permanent working electrodes.
In one aspect of the present invention, a flow-through electrochemical cell assembly is provided with a disposable working electrode structure. The assembly includes (a) a perimeter wall defining a sample flow channel including an inlet and an outlet, (b) a sample inlet line in fluid communication with the sample flow channel inlet, (c) a sample outlet line providing fluid communication between the sample flow channel outlet and a remote reference electrode, and (d) a disposable working electrode structure comprising an electrically conductive and electrochemically active working electrode region bound as a layer, directly or indirectly, to an electrically insulating substrate surface. The substrate surface is in fluid-sealing relationship with the sample flow channel, and the working electrode region is in fluid communication with said sample flow channel. The working electrode structure is readily removable from said electrochemical cell assembly.
In another aspect of the invention, a method is provided for making a disposable electrode structure and sample flow channel for such an assembly. The method comprises the steps of (a) vapor depositing electrically conductive and electrochemically active material, directly or indirectly, onto an organic polymer substrate through a mask to form a pattern of a working electrode region, and (b) forming a fluid seal between said working electrode region and a perimeter wall to define a fluid sample flow channel with said working electrode region in direct fluid contact with said fluid sample flow channel.